Image recording process

ABSTRACT

A method of imaging which comprises exposing an amorphous or crystalline chalcogenide layer to a pattern of light having wavelengths less than the band gap radiation of said chalcogenide, whereby the optical density of the layer is increased in the areas exposed to the light to form a visible image.

United States Patent Berkes et al.

IMAGE RECORDING PROCESS Inventors: John S. Berkes, Webster; William J.

Hillegas, Fairport; Samuel W. Ing, Webster, all of NY.

Assignee: Xerox Corporation, Stamford,

Conn.

Filed: Aug. 23, 1972 Appl. No.: 283,124

Published under the Trial Voluntary Protest Program on January 28. 1975as document no. B 283,124.

Related U.S. Application Data Division of Ser. No. 129,184, March 29,1971, abandoned.

US. Cl 96/27 R; 96/90 PC; 96/88 lnt. Cl. G03C 13/00 Field of Search96/27 R, 1.5, 88, 90 PC Dec. 2, 1975 [56 References Cited OTHERPUBLICATIONS C. A., 1934, 28, 4315. C. A., 1923, 17, 928. C. A., 1922,16, 2244. British Journal of Photography, 4/49, p. 188.

Primary Examiner-Charles L. Bowers, Jr.

Assistant ExaminerJohn L. Goodrow Attorney, Agent, or FirmJames J.Ralabate; James P. OSullivan; Jerome L. Jeffers 13 Claims, No DrawingsIMAGE RECORDING PROCESS This is a division of application Ser. No.129,184 filed Mar. 29, 1971 now abandoned.

BACKGROUND OF THE INVENTION This application 'relates to a novel imagerecording process.

The instant invention is somewhat related to both xerography and silverhalide photography, but has certain advantages and distinctions overboth of these imaging systems.

In the art of xerography, a xerographic plate which contains aphotoconductive insulating layer is imaged by first uniformlyelectrostatically charging the photoconductive surface. The plate isthenexposed to a pattern of activating electromagnetic radiation such aslight, which selectively dissipates the charge in the illuminated areasof the photoconductive insulator while leaving behind a latentelectrostatic image in the nonilluminated areas. This latent image maythen be developed to form a visible image by depositing a finely dividedelectroscopic marking particles on the surface of the photoconductiveinsulating layer. This concept was originally described by Carlson inU.S. Pat. No. 2,297,691 and is further amplified and described by manyrelated patents in the field.

In silver halide photography, exposure of a silver halide emulsion layerto light, even for a short time, activates the silver halide particlesso that they are much more easily reduced than before activation withthe light exposure. This latent image which is formed by the exposure tolight is then developed by washing the plate in amild reducting agent,such as hydroquinone or pyrogallol, which has the ability to reduce thesilver ions in those grainsof silver halide upon which the light hasfallen, while those grains which have not been struck with light areleft unchanged. As a result of the developing process, the image appearsupon the plate and is formed by a deposit of finely divided silver whichis proportional in thickness to the intensity of light whichfell uponeach part of the plate. Any portion of the original object which waswhite will appear black upon the image, and because of this reversal ofcolors, the plate is called a negative. Before the negative can bebrought'out in the dark room, it is exposed to a solution of sodiumthiosulfate which dissolves any unchanged silver halides, but has noaffect upon the metallic silver of the image.

Both of the imaging processes defined above have particular applicationor utility in reproducing an image, as can be seen from the descriptionof each technique. In order to produce a second or duplicate image inxerography, the entire xerographic process must be repeated a secondtime. In other words, as many duplicates as desired may be made bysimplyrepeating all the process steps for the formation of eachadditional image. On the other hand, in forming a reproduction orphotograph of an'image by silver halide photography, the process for agiven exposure is irreversible in that only one silver halide image maybe formed from a given sheet of film. It can therefore be seen that forboth xerography and silver halide photography, additional processingsteps are required after image exposure. In order to form multiple orduplicate images in both techniques, either the entire process sequencemust be repeated as in xerography; or as in silver halide 2 photography,a second or separate sheet of film must be used in order to reproduce agiven image a second time.

In addition to the above, both xerography and silver photography haveparticular limitations with regard to their ultimate imaging resolution.For example, in xerography, the particle size of the toner willestablish the resolution limit. In silver halide photography, theultimate resolution of this system is directly related to the discreteparticle size of the silver halide material.

It can be seen from the above that there exists a need for an imagingsystem in which images having high resolution can be formed by moresimple and direct techniques.

There has now been discovered an imaging recording process by whichvisible images may be formed by single exposure to imaging light with nosubsequent development of any type being required.

OBJECT OF THE INVENTION It is, therefore, an object of this invention toprovide a novel imaging system which overcomes the above noteddisadvantages.

It is another object of this invention to provide a novel imaging systemby which a visible image is formed directly from a single exposure toimaging light.

It is a further object of this invention to provide an imaging systemwhich does not require image development.

It is a further object of this invention to provide a novel imagingsystem which has recycling characteristics and which allows for imagesto be formed directly from a single exposure to imaging light.

It is another object of this invention to provide an imaging systemhaving ultra-high resolution capability.

SUMMARY OF THE. INVENTION The foregoing objects and others areaccomplished in accordance with this invention by providing an amor--phous or crystalline chalcogenide layer which undergoes a photochemicalreaction when exposed to light of wavelengths less than the band gapradiation of the particular amorphous or crystalline material. The bandgap radiation may also be described as the effective electronic energygap. This wavelength or wavelength range is the point at which a givenmaterial exhibits the onset of carrier generation. It has been foundthat when these amorphous or crystalline layers are exposed to suchradiation, a photochemical reaction occurs in the exposed areas whichchanges the reflectivity of thesurface and increases the optical densityof the film when viewed in transmission. Since the changes caused by theradiation occur in the region exposed to light, an image recordingorphotographic system results. Images formed by this technique may bestored indefinitely or may be viewed or reconstructed immediately. Thereconstruction of the image can be made with light whose wavelength iseither shorter or longer than the band gap radiation used to form theimage. Wavelengths which are longer than the band gap radiation of theparticular material are preferred in that they provide superior imagecontrast. When desiring-to use the film for an additional or addedimage, the original image may be removed by simply heating the imagingflim which effectively erases the visible image.

DETAILED DESCRIPTION OF THE INVENTION The instant invention is directedto an imaging system which utilizes the photochemical reaction whichoccurs in both amorphous and crystalline forms of various chalcogenidesystems when exposed to light in the wavelength region of less than theband gap radiation for the particular material. For purposes of thisinvention chalcogenides may comprise any materials made up of sulfides,selenides, tellurides or mixtures thereof. In addition, when in thevitreous form, these chalcogenide systems may vary widely from theirstoichiometry. The exposure to light in the form of an image results ina change of the reflectivity of the surface of the material andincreases the optical density of the film when viewed in transmission.For example, amorphous films of arsenic triselenide and arsenictrisulfide exhibit a change in surface reflectivity which is sometimesassociated with a topographical smoothing of the surface in theradiation struck areas. The illuminated area when viewed in transmittedlight exhibits an increase in optical density. Images formed by thismethod can be effectively erased by heating which renders the layerready to be imaged a second time.

1 Although the theory of the above mechanism is not fully understood, itis believed that the change in optical density in the light struck areasis the result of phase separation in the region in which light isabsorbed. For example, in arsenic triselenide, it is believed that verysmall particles of elemental arsenic are suspended in a matrix ofamorphous selenium or arsenic-enriched selenium'glass. The removal ofthe optical densified region by heating probably occurs by the processwhere all of the arsenic is taken back into solution by the selenium.This process will produce a homogeneous amorphous material identical tothe film before it was illuminated, provided no arsenic has been lost.This effect has been observed for arsenic trisulfide.

In one embodiment, thin amorphous films of the above identifiedmaterials are formed in a thickness of several microns on any suitablesupport substrate. The support may comprise any suitable conductive orinsulating material. Typical conductive materials include brass,aluminum, steel or the like. The amorphous films may be contained on arigid or flexible supports and in any suitable form such as a sheet,web, cylinder, or the like. Inasmuch as there is no necessary chemicalreaction between the amorphous film and substrate, or any necessaryelectrical requirements, it is preferred that the substrate comprise adielectric material such as glass, aluminum oxide, a ceramic, plastic,or any other suitable dielectric. If the image is to be used intransmission it is also preferred that the substrate be transparent.When used in the amorphous form, devices employing amorphous layers maycomprise any suitable chalcogenide system. Typical compositions includearsenic triselenide, cadmium selenide, gallium selenide, arsenictrisulfide, arsenic telluride, cadmium sulfide, antimony sulfide,antimony selenide, antimony telluride, bismuth sulfide, bismuthselenide, bismuth telluride, germanium sulfide, germanium selenide,germanium telluride, and mixtures thereof such as ternary, quaternaryand more complex systems defined by all combinations of these materials.For example, these might include ternaries containingarsenic-bismuthselenide or arsenic-germanium-telluride, etc.

The band gap radiation for arsenic trisulfide is about 5300 AngstromUnits, while the band gap radiation for arsenic triselenide is about7l00 Angstrom Units. For the various chalcogenide systems of the instantinvention, the band gap radiation varies from about 5000 to 10,000Angstrom Units. Since the photochemical reac tion is essentiallytemperature independent, images can be formed at all temperatures belowthe liquidus or fusion temperature and images have been formed attemperatures as low as 120K. Similarly, erasure of the image can besimply accomplished by heating the film to a temperature above thetemperature at which the image is formed. The erasure temperature neednot necessarily exceed the glass transition or liquidus temperature.

The thickness of the amorphous film is not particularly critical butthicknesses in the range of about I to 10 microns have been found toyield particularly satisfactory results. When viewed in transmissionwith visiv ble light, however, the non-imaged areas must be transparentto the reading light and consequently thinner samples may be necessaryfor materials which have a band gap in the red.

The amorphous layers of the instant invention may be prepared by anysuitable technique. A preferred technique includes vacuum evaporationwherein the composition forming the amorphous layer is evaporated onto asuitable base material or support. In general, the vacuum conditions mayvary from about 10 to 10' Torr. In one embodiment, a master alloycomplaced in separate heated crucibles and maintained.

under vacuum conditions with a source temperature of each alloyconstitutent being controlled so as to yield the appropriate percentageor proportion of the alloy desired.

Another typical method of making amorphous layers 1 of the instantinvention includes flash evaporation under vacuum conditions which aresimilar to those defined in co-evaporation. In this technique, apowdered alloy such as arsenic triselenide is selectively dropped into aheated crucible maintained at a temperature, in the range of about 300to 600C. The vapors formed from the heated mixture are evaporatedupwards onto a substrate supported above the crucible.

In all of the above evaporation techniques, the substrate onto which thephotconductive material is evaporated is usually maintained at atemperature of about 20 to C. If desired, a water cooled platen or othersuitable cooling means may be used in order to maintain a contantsubstrate temperature. In general, an amorphous arsenic selenide layerapproximately 5 microns thick is obtained when vacuum evaporation iscarried out for about 30 minutes at a vacuum of about 10' Torr and acrucible temperature of about 400C. In general, crucibles which are usedfor evaporation of amorphous layers may be of any inert material such asquartz, molybdenum, stainless steel, or stainless steel coated with alayer of silicon monoxide.

In imaging the amorphous layers of the instant inventiomthe layer mustbe exposed to light in the wavelength region which is less than the bandgap radiation. This range is required in order for the photochemicalreaction to occur and cause a change in reflectivity of the surface inthe light struck areas. In general, the greater the intensity of theexposing light. the shorter time duration required for exposure. Forexample. when using ordinary tungsten light from an optical microscope,the light intensity is rather low and hence a 'rather long time period,sometimes several hours or more, is required. On the other hand whenusing a high intensity light source such as a laser, the imaging may becarried out in a matter of seconds or minutes. It therefore follows thatas the intensity of the imaging light becomes greater, the density ofthe image increases.

Amorphous films of the instant invention can be viewed or reconstructedimmediately with no further development step being required after theinitial exposure. The reconstruction of the image can be made with lightwhose wavelength is preferably longer than the band gap radiation. Ifdesired, the image can be erased by heating the member containing theamorphous film. After erasure of the image the member is ready forsubsequent imaging. Because the entire amorphous layer is active orsensitive to light, the resolution of images formed by the technique ofthe instant invention should approach the precipitate dimensions of thematerial. This high resolution capability has utility in microrecordingsystems such as in holographic and memory devices. Resolutions of 70line pairs per millimeter have been demonstrated. The theoreticalresolution limit would be determined by the perfection of the evaporatedfilm and should be greater than about 200 line pairs per millimeter. Ingeneral, the stability of images formed by this technique when kept atroom or ambient conditions will not normally deteriorate in any knowntime period. For example, images have been made and stored for IS monthswith no degradation in image quality being observed.

When using samples of crystalline chalcogenides, the samples may beprepared by any conventional techniques which are used for formingcrystalline materials. For example, crystals of arsenic triselenide maybe formed by first taking a master alloy having the appropriate ratio ofarsenic and selenium (i.e. As se and placing the master alloy in asealed quartz ampul. The ampul containing the alloy is placed in an ovenmaintained at 336C. for 7 days. At the end of this time the ampul iscooled, broken open, and a number of arsenic triselenide crystals whichhave been formed by vapor transport, are removed from the ampul.Crystals of arsenic sulfide are formed by the above method by heating ata temperature of 250C. In addition, 1.0 percent thallium isadded to thealloy in order to insure crystal formation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples furtherspecifically define the present invention with respect to a method ofmaking and imaging amorphous films of chalcogenide systems.

EXAMPLE I sealed. The sealed ampul is then placed into a furnace whichis heated to a temperature of about 600C. in a period of about 3 hours.To aid in the reaction of the aresenic and selenium and to assure alloyuniformity, the furnace is rocked for about a 12 hour period at 600Cafter which the ampul is removed from the furnace and quenched in waterat room temperature. The alloy is then removed from the ampul, crushed,and is then ready for use as a master alloy of arsenic triselenide forvacuum evaporation.

EXAMPLE 2 A master alloy of arsenic trisulfide is formed by the methodof Example 1 using a ratio of 40 atomic percent arsenic and 60 atomicpercent sulfur.

EXAMPLE 3 A series of three amorphous arsenic triselenide films I to 2microns thick contained on a pyrex glass substrate are made by thefollowing technique. A sample of the master alloy of arsenic triselenideformed by Example 1 is placed in a stainless steel boat which is insorted into a vacuum chamber. A pyrex glass substrate is suspended about12 inches above the boat and the chamber evacuated to a vacuum of about10 Torr. The stainless steel boat containing the arsenic triselenide isthen heated to a temperature of about 400C. for about 10 minutes. Thesubstrate is maintained at a temperature of about 20 to 25C. during theevapora tion process. At the end of the evaporation cycle, the

vacuum chamber is cooled to room temperature, the

vacuum broken, and the pyrex substrate containing an amorphous filmabout I to 2 microns thick of arsenic triselenide is removed from thechamber. This process is repeated two additional times to form a totalof three arsenic triselenide plates contained on pyrex glass substrates.A fourth plate is made as above using an aluminum oxide wafer as thesubstrate.

EXAMPLE 4 The method of Example 3 is used to form a 5 micron thick layerof arsenic trisulfide contained on an aluminum oxide wafer. Thestainless; steel crucible is heated to a temperature of about 350C.

EXAMPLE 5 A plate formed by Example 3 which contains an amorphous filmor amorphous arsenic triselenide contained on the pyrex glass substrateis imaged by the following technique. A test pattern is formed byprojection onto the arsenic triselenide surface by using a light sourcecontaining a tungsten lamp having at 20 watt output. The light isfocused onto the amorphous surface and a test pattern projected at 1:1magnification onto the arsenic triselenide film with f/2.8 optics. Theprojection method is similar to that used for a standard slideprojector. This image technique is carried out for an exposure 1 hour atunder atmospheric conditions at a temperature of about 23C. Thisresultant image is stable and exhibited no noticeable deteriorationafter 18 months of storage under room ambient conditions in a lighttight container. The image quality of the test pattern was sharp and hasa resolution of about line pairs per millimeter when observed through amicroscope.

EXAMPLE 6 A second sample formed by Example 3 containing an amorphousarsenic triselenide film contained on pyrex glass is imaged by contactprinting using the following technique. A light image is formed byfocusing a microscope light using a tungsten light source from a LeitzOrtholux Microscope using a x objective. The exposure is for a timeperiod of minutes under atmospheric conditions at a temperature of 23C.

The image of the letter x from the contact image source is recorded onthe film. When viewed in transmitted visible light this photographappears as a black letter on a lighter background.

EXAMPLE 7 Using a 5 micron film of arsenic trisulfide contained on analuminum oxide wafer substrate (Example 4), a contact print was formedby the following technique. A light image was formed by focusingmicroscope light from a tungsten filament using a Leitz Microscope witha 5x objective. The exposure time is for a period of 2 hours underatmospheric conditions at a temperature of 23C.

The image is similar to that described in Example 6 above.

EXAMPLE 8 A sample containing an amorphous layer of arsenic triselenidecontained on the pyrex glass substrate (Example 3) is imaged by a laserbeam using the following technique. The laser beam contains a lightsource of 4579 Angstrom Units and a light intensity of 10 (photons/cmsec.) using an exposure time of 3 seconds at a temperature of 25C underatmospheric conditions. A clearly visible image consisting of a darkspot the size of the laser beam (about 2mm in diameter) is visible whenthe filter is viewed in transmitted visible light.

EXAMPLE 9 A sample containing a 1 micron layer of arsenic triselenidecontained on an aluminum oxide substrate wafer (Example 3) are imaged bya laser beam as follows. The laser beam has a wavelength of 6328 AnstromUnits and a light intensity of 10 (photons/cm seconds) at a temperatureof l20K under a vacuum of 10" centimeters of mercury. An exposure timeof 20 seconds results in a relative transparency of 0.75 compared to thevirgin material.

In addition to the above imaging, the image can be erased by heating thesample to 250K and subsequent cooling to 120K. The same results can beobtained by carrying out the initial exposure at 298K. Erasure can beachieved by heating to 400K and subsequently cooling back to 298K. Thereaction responsible to the reversibility is at least as fast as theheating rate (about 1K/min.) which is used in the experiment.

EXAMPLE 10 An arsenic trisulfide crystal having a needlelike shape about3 millimeters long and a cross-section of about 0.2 millimeters isilluminated by focusing the microscope light on a portion of the crystalby the illumination technique described in Example 7. The crystal isexposed for about 12 hours which results in the formation of a dark spotabout 0.3 millimeters long when the crytal is viewed inreflected light.This experiment is repeated using an arsenic triselenide crystal. Theresults 8 are similar to those observed for the arsenic trisulfidecrystal.

Although specific components and been stated in the above description ofthe preferred embodiments of this invention, other suitable materialsand procedures such as those listed above may be used with similarresults. In addition, other materials and changes may be utilized whichsynergize, enhance, or otherwise modify the above techniques.

Other modifications and ramifications of the present invention wouldappear to those skilled in the art upon reading the disclosure. Theseare also intended to be within the scope of this invention.

What is claimed is:

l. A method of imaging which comprises exposing an amorphous layer ofarsenic trisulfide to a pattern of light having wavelengths less thanthe bandgap radiation of said arsenic trisulfide, whereby the opticalden sity of the amorphous layer is increased in the areas exposed to thelight to form a visible image and heating the imaged layer to atemperature above that at which the image was formed to erase the image.

2. A method of imaging which comprises exposing a crystalline layer ofarsenic triselenide to a pattern of light having wavelengths less thanthe bandgap radiation of said arsenic triselenide, whereby the densityof the crystalline layer is increased in the areas exposed to i 4. Themethod of claim 1 in which the amorphouslayer is contained on thedielectric substrate.

5. The method of claim 1 wherein the amorphous layer is contained on adielectric substrate.

6. The method of claim 2 wherein the light has wavelengths of less than7100 Angstrom units.

7. The method of claim 2 wherein the image is erased by heating theamorphous layer to a temperature in the range of about l00 to about120C. greater than the exposure temperature.

8. The method of claim 1 in which the image is erased by heating theamorphous layer to a temperature in the range of about to C greater thanthe exposure temperature.

9. The method of claim 2 wherein the crystalline layer is contained on adielectric substrate.

10. A method of imaging which comprises exposing a layer of an amorphousor crystalline chalcogenide material selected from the group ofsulfides, selenides or tellurides of arsenic, antimony, bismuth, galliumor germanium to a pattern of light having wavelengths of less than thebandgap radiation of said chalcogenide, said light having sufficientintensity to cause a photochemical reaction in the light struck areas,whereby the optical density of the layer is increased in the areasexposed to light resulting in the formation of a visible image andheating the imaged layer to a temperature above that at which the imagewas formed to erase the image.

11. The method of claim 9 wherein the chalcogenide is selected from thegroup of arsenic triselenide, cadmium selenide, gallium selenide,arsenic trisulfide, arsenic telluride, cadmium sulfide, antimonysulfide, antimony selenide, antimony telluride, bismuth sulfide, bismuthselenide, bismuth telluride, germanium sulfide, germanium selenide,germanium telluride and mixtures thereof.

proportions have 12. The method of claim 9 in which the layer isconrange of about to 120C greater than the expo tained on a dielectricsubstrate.

13. The method of claim 9 wherein the image is sure temperatureerased byheating the layer to a temperature in the

1. A METHOD OF IMAGING WHICH COMPRISES EXPOSING AN AMORPHOUS LAYER OFARSENIC TRISULFIDE TO A PATTERN OF LIGHT HAVING WAVELENGTHS LESS THANTHE BANDGAP RADIATION OF SAID ARSENIC TRISULFIDE, WHEREBY THE OPTICALDENSITY OF THE AMORPHOUS LAYER IS INCREASED IN THE AREAS EXPOSED TO THELIGHT TO FORM A VISIBLE IMAGE AND HEATING THE IMAGED LAYER TO ATEMPERATURE ABOVE THAT AT WHICH THE IMAGE WAS FORMED TO ERASE THE IMAGE.2. A method of imaging which comprises exposing a crystalline layer ofarsenic triselenide to a pattern of light having wavelengths less thanthe bandgap radiation of said arsenic triselenide, whereby the densityof the crystalline layer is increased in the areas exposed to the lightto form a visible image and heating the imaged layer to a temperatureabove that at which the image was formed to erase the image.
 3. Themethod of claim 1 wherein the light has wavelengths of less than 5300Angstrom units.
 4. The method of claim 1 in which the amorphous layer iscontained on the dielectric substrate.
 5. The method of claim 1 whereinthe amorphous layer is contained on a dielectric substrate.
 6. Themethod of claim 2 wherein the light has wavelengths of less than 7100Angstrom units.
 7. The method of claim 2 wherein the image is erased byheating the amorphous layer to a temperature in the range of about 100*to about 120*C. greater than the exposure temperature.
 8. The method ofclaim 1 in which the image is erased by heating the amorphous layer to atemperature in the range of about 100 to 120*C greater than the exposuretemperature.
 9. The method of claim 2 wherein the crystalline layer iscontained on a dielectric substrate.
 10. A method of imaging whichcomprises exposing a layer of an amorphous or crystalline chalcogenidematerial selected from the group of sulfides, selenides or tellurides ofarsenic, antimony, bismuth, gallium or germanium to a pattern of lighthaving wavelengths of less than the bandgap radiation of saidchalcogenide, said light having sufficient intensity to cause aphotochemical reaction in the light struck areas, whereby the opticaldensity of the layer is increased in the areas exposed to lightresulting in the formation of a visible image and heating the imagedlayer to a temperature above that at which the image was formed to erasethe image.
 11. The method of claim 9 wherein the chalcogenide isselected from the group of arsenic triselenide, cadmium selenide,gallium selenide, arsenic trisulfide, arsenic telluride, cadmiumsUlfide, antimony sulfide, antimony selenide, antimony telluride,bismuth sulfide, bismuth selenide, bismuth telluride, germanium sulfide,germanium selenide, germanium telluride and mixtures thereof.
 12. Themethod of claim 9 in which the layer is contained on a dielectricsubstrate.
 13. The method of claim 9 wherein the image is erased byheating the layer to a temperature in the range of about 100* to 120*C.greater than the exposure temperature.